Hi,
I am not sure if it is possible to override filters - check it out.
However, I would suggest not doing so - even if that is allowed.
Predefined filters are the public API of AngularJS. What if some parts
of AngularJS use some of them internally or one day you install some
add-on which depends on that filter?

By the way: there is no need to inject $filter, you can specify which
exactly filters you want on injection point, so you are more specific
about real dependencies:
myApp.filter('myDate', function(dateFilter){
return function(date){
return 'prefix ' + dateFilter(date, 'yyyyMMddhhmmss');
}
});